Comprehensive Phytochemical Characterization of Echinophora tenuifolia subsp. sibthorpiana Reveals Radical Scavenging, Antimicrobial, and Antiproliferative Activities Associated with Apoptosis and G2/M Cell Cycle Accumulation

Echinophora tenuifolia subsp. sibthorpiana is an aromatic and traditionally used plant whose phytochemical diversity and pharmacological potential remain incompletely characterized. This study aimed to integrate comprehensive chemical profiling with antioxidant, antimicrobial, and antiproliferative evaluations of an acidified methanolic extract prepared from its aerial parts. Phenolic constituents, volatile compounds, and fatty acids were characterized using LC–MS/MS, GC–MS, and GC–FID, respectively. Antioxidant capacity was assessed using DPPH and ABTS assays, antimicrobial effects were evaluated by disc diffusion and broth microdilution methods, and antiproliferative activity was investigated in six human cancer cell lines and non-cancerous BEAS-2B cells. Apoptosis induction and cell-cycle alterations were further examined by flow cytometry. The extract contained a high total phenolic content of 181.78 ± 5.03 mg GAE/g and exhibited substantial DPPH and ABTS radical scavenging activities of 333.65 ± 2.94 and 262.18 ± 6.80 mg TE/g, respectively. LC–MS/MS analysis identified rutin hydrate, quinic acid, chlorogenic acid, narcissin, and vicenin-2 as the predominant non-volatile constituents. The essential oil was dominated by methyl eugenol (51.10%) and Δ-3-carene (32.23%), whereas lauric acid (29.71%) was the major fatty acid. The extract also exhibited measurable antimicrobial activity and concentration-dependent cytotoxicity, with the greatest sensitivity observed in MCF-7 breast cancer cells (IC50 = 32.96 ± 4.80 μg/mL), followed by A549 and MDA-MB-231 cells. However, the relatively close IC50 values observed in cancer and non-cancerous BEAS-2B cells indicated limited cancer-cell selectivity under the tested conditions. Flow-cytometric analyses demonstrated marked apoptosis induction in MCF-7 cells, reaching 57.93 ± 1.80%, together with a cell-line-dependent accumulation in the G2/M phase. Collectively, these findings indicate that E. tenuifolia subsp. sibthorpiana is a chemically diverse source of plant-derived bioactive compounds with notable radical scavenging and measurable antimicrobial and cytotoxic effects. Further bioactivity-guided fractionation and mechanistic studies are required to identify the active constituents and clarify their pharmacological relevance and selectivity.